1. Field of the Invention
This invention is in the field of electrical generator and motor structures, including specific fields relating to brushless alternator structures and brushless synchronous-motor structures. Specific fields of the invention also include permanent-magnet, motor-generator devices and include methods for construction of those devices without using permanent magnets.
2. Description of the Prior Art
Prior-art synchronous alternators and electrical motors are, in commercially successful form, constructed with rotors and stators in which hollow, cylindrical, laminated, magnetic cores of the stators surround the magnetic portions of the rotors. The excitation field windings of alternators and of excitation-type motors are usually mounted on the rotors of such prior-art devices. Energizing of the field windings is typically accomplished through use of brush connections that conduct current from and to direct-current electrical-energy sources that are external to the rotors or, in some brushless machines, through direct-current electrical-energy sources fabricated within the rotor structures. Structures having either brushes or internal rotor field energizing circuits require a relatively large number of parts and, therefore, increased cost of manufacture. Maintenance costs are also increased because of the limited life span of brushes and other parts.
Permanent-magnet rotors, rather than energized rotor field windings, are sometimes used in alternator and in motor applications where transient loads or adverse environmental conditions do not cause demagnetization. Varying-reluctance rotors with no excitation field are sometimes used for synchronous-motor applications having relatively low torque requirements. Prior-art structures having either permanent-magnet rotors or varying-reluctance rotors are, in general, also constructed with rotors surrounded by laminated stator cores.
The stator magnetic fields of conventional synchronous machines are associated with phased alternating-currents flowing through windings mounted on the stators. The constant-rotating-magnitude magnetic fields associated with stator windings typically enter and leave the laminated stator cores at separate fixed locations on the cylindrical inner surfaces of the cores, requiring field paths having tangential direction through the stator cores. The rotating fields associated with rotor-mounted windings and permanent-magnet rotors enter and leave the stator cores at separate rotating locations on the cylindrical inner surfaces, also following tangential paths through the cores. Therefore, the stator cores of prior-art structures must have radial thicknesses of sufficient dimension that the combined rotor and stator magnetic fields do not saturate while moving in tangential paths within the cores.
Hysteresis losses in the ferrous stator cores of commercially successful prior-art devices are relatively large because the combined rotor and stator magnetic fields reverse direction at every point in the stators with each cycle of alternating-current and corresponding rotation of the rotors. The hysteresis losses per unit volume would be decreased if the magnetic fields in the stator cores did not reverse directions, but varied in only one direction between a minimum value and a maximum value during each cycle, thus avoiding operation involving the major hysteresis loops of the magnetic materials.
While prior-art disclosures include brushless alternator and synchronous-motor structures having stationary excitation windings that produce magnetic fields associated with the rotors, those prior-art structures have not been commercially successful, apparently because of expensive and complex magnetic circuitry or because of adverse eddy-current effects in less complex magnetic circuitry. No known prior-art structure having a stationary excitation winding has disclosed a rotor-stator combination that minimizes adverse effects of eddy currents and hysteresis in both stator and rotor through use of a simple and energy-efficient design in which radial, time-varying magnetic fields need not reverse direction in the stator and in which a laminated stator core of simple construction may have a volume equivalent to that of conventional machines with similar operating specifications and with similar construction materials.